The present invention relates to a method and apparatus for determining the imbalance of a rotational member, particularly a wheel, and more specifically to a method and apparatus that is capable of dynamically determining the imbalance of a rotational member in a simple and economic manner. Although it is discussed hereinafter with particular reference to the balancing of wheels, it will be appreciated that the invention is applicable to other types of rotational members, e.g. rotors or the like. It will also be appreciated that when this application refers to balancing "wheels" what is actually being balanced is usually a wheel-tire combination, and that "wheel" includes this combination.
At present, there are available two basic types of systems for determining the imbalance of a wheel having a tire mounted thereon. One of these types of systems is a static one, in which the wheel and tire remain motionless while any imbalance thereof is determined. For example, the wheel can be supported in a horizontal orientation by means of a leveling type of support device having a bubble or similar such indicator that is shifted from a central reference point in dependence upon the imbalance of the tire. By placing suitable weights at strategic points around the rim of the wheel, an operator is able to vary the position of the bubble indicator until it is brought to the reference point. Thereafter, the weights are fixed to the wheel at these strategic points.
The primary advantages of a static type of wheel balancing system lie in its simplicity and low cost. Due to its relatively simple construction and small size, it is an easily affordable system that can be found in a variety of establishments in which wheel balancing is required on a part-time basis, such as in automotive shops, service stations, and the like. However, the static type of wheel balancing system is less than totally satisfactory. Since the wheel is motionless in a static system, the imbalance that is determined basically relates only to the location of the center of gravity relative to the spin axis. However, the state of balance of the wheel is most critical when it undergoes rotation. It is only then that possibly counterbalancing centrifugal forces present on the inner and outer planes of the wheel can be detected and distinquished.
Consequently, the second type of wheel balancing system, a dynamic balancing system, has been developed to provide a more complete indication of the imbalance of a wheel and the manner in which it can be corrected. In dynamic wheel balancing systems, the wheel to be balanced is mounted on a shaft that is brought up to a desired rotational speed by means of a motor. Once the wheel attains this speed, measurements are made of the lateral movement of the shaft, or the forces generated by the shaft, due to the imbalance of the wheel. From these measurements, indications of the magnitude of the imbalance of the wheel and the location of the imbalance can be made.
Typically, the rotational speed of the wheel is 400 rpm or greater. Such a speed has been used for several reasons. For one, it is easier to measure the forces of imbalance at high speed as the magnitude of the imbalance forces increases as the square of rotational velocity. At high velocities very favorable signal/noise ratios are easily obtained.
An additional reason that these high rotational velocities have been used in the past has related to the consumer's observation that imbalanced tires are more noticeable at high highway speed than at low speeds. This has led to the generally held belief that it is necessary that wheel and tire balancing be carried out at the wheel's usual operating speed. A passenger car tire rotates at about 400 rpm at 30 mph and about 750 rpm at 55 mph.
Although the presently available high speed dynamic wheel balancing systems provide a highly accurate reading of the imbalance of a wheel, as well as the location and amount of weight necessary to correct it, they are also not without their attendant limitations. Foremost among these is the cost of such systems. One of the major contributors to this cost is the drive motor and associated heavy duty drive train that are incorporated in such systems and are necessary to bring the wheel up to the measuring speed heretofore deemed appropriate.
Furthermore, the high speed at which the wheel is rotated during the balancing operation presents a number of safety hazards that must be compensated. For example, industry practice requires that wheel balancers of this type have a protective hood mechanism that covers a substantial portion of the wheel while it is rotating, to eliminate the hazards caused by objects that are trapped within the treads of the tire being loosened and flying off, and thereby injuring someone. The hood also lessens the likelihood that the operator of the balancing mechanism will get his hands, hair or clothes caught in the wheel or on the treads and thereby be injured during the rotation of the wheel, or that the wheel will come off. This hood also has an interlock which prevents the balancer from operating unless it is firmly seated. In addition to the hood, it is also necessary to provide a brake on the balancing machine that is of sufficient strength to stop the wheel quickly, even at high rotational speeds, before the hood can be lifted.
All of the components that are required to make up a dynamic wheel balancing system of the type that is presently commercially available result in a machine that is quite large in size. Since the forces generated at high speeds can be very large these machines are heavily weighted and/or bolted to the floor. This is also needed for safety. They are not generally considered to be portable in nature. The complexity of heretofore known dynamic balancing machines renders the serviceability of such machines much more difficult. The wear occasioned by the high operating speeds and the need to be able to quickly decelerate the rotating wheels increases the frequency of required maintenance, thereby also increasing the real cost of such machines.